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1.
There is increasing evidence that the nature of extended dark features on slopes of Martian craters and uplands is related to existing sources of liquid water located on these slopes and to confined water flows rather than to the movement of large masses of dry sand (dust) or rock falls. Images acquired by the Mars Global Surveyor Mars Orbiter Camera at spatial resolutions of a few meters per pixel make it possible to distinguish such objects. The availability of big reserves of ground ice on Mars and conditions for the local conversion of ice to the liquid phase is now universally accepted. Although the presence of liquid water on the Martian surface is usually thought to be impossible because of low pressures and low mean temperatures, there is a sufficient number of lowlands on Mars where pressure exceeds the critical value required for the existence of liquid water. The extended narrow gullies on slopes with tributaries were formed, as it is supposed, by water streams. The structure of gullies has an unusual appearance, reverse of that of mountain rivers on Earth: gullies are broad in the upper part of a slope, narrow downslope, end with a thin stream, and disappear at the valley or crater floor. Both tributaries and the major channel seem to be directed uphill. This paper provides a simple explanation of this apparent paradox. Under low-temperature conditions, the conversion of liquid water to the ice phase should be considered in dynamics: the water released by the source comes in contact with a cold ground, partly soaks in ground, and freezes, forming an ice bed along which the stream moves further and continues to interact with ground. The distance from the source at which water completely disappears depends on the initial temperature of the source, its abundance, and the ground temperature. The apparent paradox is explained by the interaction of a cooling stream with a very cold ground. As regards the side structures, they are not tributaries but branches, which rapidly freeze. This paper also shows that a high source debit and/or sufficiently high ambient temperature promote the formation on the valley floor of a small pond that accumulates water flows. The walls of this pond consist of frozen ground and ice. Objects that might be small water reservoirs are detected in some new images of Mars. High concentration of sources of groundwater in two equatorial regions of Mars may serve as a useful indication to the location of places promising for searching traces of life on this planet. 相似文献
2.
Lakes on Mars were formed under periglacial to glacial climates. Extreme conditions prevailed including freezing temperatures, low atmospheric pressure, high evaporation/sublimation rates, and liquid water reservoirs locked in aquifers below a thick cryosphere. Although many of the Martian paleolakes display evidence of a short period of activity consistent with these conditions, others display clear evidence of lifetimes ranging from 104 to 105 years. The discovery of young seeping processes in impact craters and pole-facing valley slopes along with young volcanic activity raise questions about the conditions and limitations of liquid water flow and potential lacustrine activity today on Mars. Current climate models show that in today's conditions there exist regions on Mars of sols above the triple point and below boiling point of water that could provide hydrogeological conditions comparable to these of the Antarctic Dry Valley lakes (with the exception of the atmosphere pressure). The locations of the most recent Martian paleolakes are correlated with these regions. Throughout the history of Mars, lakes generated diversified environments, which could have provided potential habitats for life. The recent discovery of young energy sources from volcanism and the potential for liquid water reinforces the possibility of extant life on Mars, and suggests recent ponds and ancient paleolakes as primary targets for rover and sample return missions. 相似文献
3.
The evaporation rate of water ice on the surface of a planet with an atmosphere involves an equilibrium between solar heating and radiative and evaporative cooling of the ice layer. The thickness of the ice is governed principally by the solar flux which penetrates the ice layer and then is conducted back to the surface. These calculations differ from those of Lingenfelter et al. [(1968) Science161, 266–269] for putative lunar channels in including the effect of the atmosphere. Evaporation from the surface is governed by two physical phenomena: wind and free convection. In the former case, water vapor diffuses from the surface of the ice through a lamonar boundary layer and then is carried away by eddy diffusion above, provided by the wind. The latter case, in the absence of wind, is similar, except that the eddy diffusion is caused by the lower density of water vapor than the Martian atmosphere. For mean Martian insolations the evaporation rate above the ice is ～ 10?8 g cm?2 sec?1. Thus, even under present Martian conditions a flowing channel of liquid water will be covered with ice which evaporates sufficiently slowly that the water below can flow for hundreds of kilometers even with quite modest discharges. Evaporation rates are calculated for a wide range of frictional velocities, atmospheric pressures, and insolations and it seems clear that at least some subset of observed Martian channels may have formed as ice-choked rivers. Typical equilibrium thicknesses of such ice covers are ～ 10 to 30 m; typical surface temperatures are 210 to 235°K. Ice-covered channels or lakes on Mars today may be of substantial biological interest. Ice is a sufficiently poor conductor of heat that sunlight which penetrates it can cause melting to a depth of several meters or more. Because the obliquity of Mars can vary up to some 35°, the increased polar heating at such times seems able to cause subsurface melting of the ice caps to a depth which corresponds to the observed lamina thickness and may be responsible for the morphology of these polar features. 相似文献
4.
Within the numerical general-circulation model of the Martian atmosphere MAOAM (Martian Atmosphere: Observation and Modeling), we have developed the water cycle block, which is an essential component of modern general circulation models of the Martian atmosphere. The MAOAM model has a spectral dynamic core and successfully predicts the temperature regime on Mars through the use of physical parameterizations typical of both terrestrial and Martian models. We have achieved stable computation for three Martian years, while maintaining a conservative advection scheme taking into account the water–ice phase transitions, water exchange between the atmosphere and surface, and corrections for the vertical velocities of ice particles due to sedimentation. The studies show a strong dependence of the amount of water that is actively involved in the water cycle on the initial data, model temperatures, and the mechanism of water exchange between the atmosphere and the surface. The general pattern and seasonal asymmetry of the water cycle depends on the size of ice particles, the albedo, and the thermal inertia of the planet’s surface. One of the modeling tasks, which results from a comparison of the model data with those of the TES experiment on board Mars Global Surveyor, is the increase in the total mass of water vapor in the model in the aphelion season and decrease in the mass of water ice clouds at the poles. The surface evaporation scheme, which takes into account the turbulent rise of water vapor, on the one hand, leads to the most complete evaporation of ice from the surface in the summer season in the northern hemisphere and, on the other hand, supersaturates the atmosphere with ice due to the vigorous evaporation, which leads to worse consistency between the amount of the precipitated atmospheric ice and the experimental data. The full evaporation of ice from the surface increases the model sensitivity to the size of the polar cap; therefore, the increase in the latter leads to better results. The use of a more accurate dust scenario changes the model temperatures, which also strongly affects the water cycle. 相似文献
5.
Outflow channeling and associated chaotic terrain were created under temperature and pressure conditions suggested for a diluvian period on Mars 3.5 to 0.5 by ago. Pressures under which both features were formed ranged from 130 to 34 mbar at a constant ambient temperature of 266°K. Analogs of the collapse structures and channels evolved in a high-altitude/low-temperature chamber are found on the Martian surface. Similarities exist not only in their overall morphology but in the finer details of the megastructures themselves. The critical factor that allowed channelized flow to occur was the sudden release of liquid water derived from melting of subsurface ground ice and ice layers under the low atmospheric pressure and temperature conditions within the chamber. Experimentation may indicate the existence of substantially thick water ice layers beneath the Martian regolith prior to the outflow channeling episode. 相似文献
6.
《Icarus》1987,72(1):95-127
The possibility that snowmelt could have provided liquid water for valley network formation early in the history of Mars is investigated using an optical-thermal model developed for dusty snowpacks at temperate latitudes. The heating of the postulated snow is assumed to be driven primarily by the absorption of solar radiation during clear sky conditions. Radiative heating rates are predicted as a function of depth and shown to be sensitive to the dust concentration and the size of the ice grains while the thermal conductivity is controlled by temperature, atmospheric pressure, and bulk density. Rates of metamorphism indicate that fresh fine-grained snow on Mars would evolve into moderately coarse snow during a single summer season. Results from global climate models are used to constrain the mean-annual surface temperatures for snow and the atmospheric exchange terms in the surface energy balance. Mean-annual temperatures within Martian snowpacks fail to reach the melting point for all atmospheric pressures below 1000 mbar despite a predicted temperature enhancement beneath the surface of the snowpacks. When seasonal and diurnal variations in the incident solar flux are included in the model, melting occurs at midday during the summer for a wide range of snow types and atmospheric pressures if the dust levels in the snow exceed 100 ppmw (parts per million by weight). The optimum dust concentration appears to be about 1000 ppmw. With this dust load, melting can occur in the upper few centimeters of a dense coarse-grained snow at atmospheric pressures as low as 7 mbar. Snowpack thickness and the thermal conductivity of the underlying substrate determine whether the generated snow-melt can penetrate to the snowpack base, survive basal ice formation, and subsequently become available for runoff. Under favorable conditions, liquid water becomes available for runoff at atmospheric pressures as low as 30 to 100 mbar if the substrate is composed of regolith, as is expected in the ancient cratered terrain of Mars. 相似文献
7.
Steven W. Squyres 《Icarus》1978,34(3):600-613
Viking orbital photographs of two regions of Martian fretted terrain have revealed a number of landforms which appear to possess distinct flow lineations. These range from valley floors with lineations which parallel the valley walls to debris aprons with distinctly lobate profiles and lineations which radiate outward from the source area. These features are attributed to the deformation and flow of a mass consisting of erosional particles and ice incorporated from the atmosphere. Such a flow should behave much like a terrestrial rock glacier. A plastic deformation model is presented which is consistent with the known mechanical properties of rock glaciers and with the observed features of the landforms. The valley floor lineations are interpreted as being due to compressional forces resulting from debris flowing inward from the valley walls. Climatic implications of the features are discussed. 相似文献
8.
Benton C. Clark 《Icarus》1978,34(3):645-665
Converging lines of evidence suggest that a significant portion of the Martian surface fines may consist of salts and smectite clays. Salts can form stoichiometric hydrates as well as eutectic solutions with depressed freezing points; clays contain bound water of constitution and adsorb significant quantities of water from the vapor phase. The formation of ice may be suppressed by these minerals in some regions on Mars, and their presence in abundance would imply important consequences for atmospheric and geologic processes and the prospects for exobiology. 相似文献
9.
Diedrich T.F. Möhlmann 《Icarus》2008,195(1):131-139
Microscopic liquid layers of water can evolve via adsorption on grain and mineral surfaces at and in the soil of the surface of Mars. The upper parts of these layers will start to freeze at temperatures clearly below the freezing point of bulk water (freezing point depression). A sandwich structure with layers of ice (top), liquid water (in between) and mineral surface (bottom) can evolve. The properties of the interfacial water (of adsorption water and premelted ice) on grain surfaces are described by a sandwich-model of a layer of liquid-like adsorption water between the adsorbing mineral surface layer and an upper ice layer. It is shown that the thickness or number of mono-layers of the interfacial water (of adsorption water and premelted ice) depends on temperature and atmospheric relative humidity. The derived equations for the sandwich model fit well to a known phenomenological relation between thickness of the liquid layer and relative humidity, and can be a tool to estimate or to determine for appropriate materials Hamaker's constant for van der Waals interactions on grains and in porous media. The curvature of grain surfaces is shown to have no remarkable effects for particles in the μm-range and larger. The application of these equations to thermo-physical conditions on Mars shows that the thickness of frost-layers, which can evolve over several hours on cooling surface parts of Mars, is typically of the order or a few tenths of one millimeter or less. This is in agreement with observations. Furthermore, an equation is derived, which relates the freezing point depression for van der Waals force governed interfacial water to the value of the Hamaker constant, to the latent heat of solidification, to the mass density of water ice, and to the thickness of the liquid-like layer. Again, this equation fits well to a known phenomenological relation between freezing point depression and thickness of the liquid-like layer. The derived equation shows that the lower limiting temperature of the liquid phase can reach about 180 K under martian conditions having an atmospheric water content of around 10 pr μm. An “Equilibrium Moisture Content” (EMC)/“Equilibrium Relative Humidity” (ERH) relation for the water content of martian soil has been derived, which relates, for equilibrium conditions, soil water content and atmospheric relative humidity. This relation indicates that the content of liquid interfacial water in the upper surface of Mars can reach up to 10% by weight and more in course of saturation during night hours, and it can be of about 2% by weight during the dry daytime hours. 相似文献
10.
All Martian channels and valleys visible at a resolution of 125 to 300 meters between 65°N and 65°S were mapped at a scale of 1:5,000,000 and the maps then digitized. Correlations of valley presence with other surface features show that almost all valleys are in the old cratered terrain. preferentially in areas of low albedo, low violet/red ratios, and high elevation. The networks are open, the individual drainage basins are small relative to Earth, and large distances separate the basins, features which all suggest an immature drainage system. The simplest explanation of the correlations and the restriction of valley networks to old terrain is that the channels themselves are old, and that the climatic conditions necessary for their formation did not prevail for long after the decline in the cratering rate around 3.9 billion years ago. Two types of outflow channel are distinguished: unconfined, in which broad swaths of terrain are scoured, and confined, in which flow is restricted to discrete channels. The outflow channels have a wide range of ages and may form under present climatic conditions. Fretted channels are largely restrited to two latitude belts centered on 40°N and 45°S, where relatively rapid erosion along escarpments results from mass wasting. They probably form by enlargement of preexisting channels by escarpment retreat. 相似文献
11.
A theoretical reconstruction of the history of Martian volatiles indicates that Mars probably possessed a substantial reducing atmosphere at the outset of its history and that its present tenous and more oxidized atmosphere is the result of extensive chemical evolution. As a consequence, it is probable that Martian atmospheric chemical conditions, now hostile with respect to abiotic organic synthesis in the gas phase, were initially favorable. Evidence indicating the chronology and degradational history of Martian surface features, surface mineralogy, bulk volatile content, internal mass distribution, and thermal history suggests that Mars catastrophically developed a substantial reducing atmosphere as the result of rapid accretion. This atmosphere probably persisted—despite the direct and indirect effects of hydrogen escape—for a geologically short time interval during, and immediately following, Martian accretion. That was the only portion of Martian history when the atmospheric environment could have been chemically suited for organic synthesis in the gas phase. Subsequent gradual degrassing of the Martian interior throughout Martian history could not sustain a reducing atmosphere due to the low intensity of planet-wide orogenic activity and the short atmospheric mean residence time of hydrogen on Mars. During the post-accretion history of Mars, the combined effects of planetary hydrogen escape, solar-wind sweeping, and reincorporation of volatiles into the Martian surface produced and maintained the present atmosphere. 相似文献
12.
Methods for calculating heat and erosion impact caused by particles in the Martian atmosphere on the heat protection of the descent module EXOMARS-2 during descent in the atmosphere are presented. Atmosphere models corresponding to climatic conditions when landing on the Martian surface are investigated for the landing site Oxita Planum. 相似文献
13.
An atmospheric general circulation model is run with boundary conditions representing different amounts of equator-to-pole oceanic heat transport. Oceanic heat transport underneath sea ice is held fixed, minimizing positive feedbacks due to sea ice and thereby providing a lower bound on the effects of oceanic heat transport on climate. When oceanic heat transport is reduced, some compensating increases in atmospheric heat transport occur, but tropical surface temperatures increase and atmospheric circulation and precipitation patterns undergo significant changes. We conclude that the ability of the oceans to generate past and future climatic changes through transport of heat is substantial, even though it is limited by a tendency of the atmosphere to partly compensate for changes in oceanic heat transport. 相似文献
14.
George Ohring 《Icarus》1975,24(3):388-394
The concept is described of deducing the temperature and constituent profile of a planetary atmosphere from orbiter measurements of the planet's ir limb radiance profile. Expressions are derived for the weighting functions associated with the limb radiance profile for a Goody random band model. Analysis of the weighting functions for the Martian atmosphere indicates that a limb radiance profile in the 15 μm CO2 band can be used to determine the Martian atmospheric temperature profile from 20 to 60 km. Simulation of the Martian limb radiance profile in the rotational water vapor band indicates that Martian water vapor mixing ratios can be inferred from limb radiance observations in a water vapor band. 相似文献
15.
James L. Fastook James W. Head Francois Forget Jean-Baptiste Madeleine David R. Marchant 《Icarus》2011,216(1):23-39
A fretted valley system on Mars located at the northern mid-latitude dichotomy boundary contains lineated valley fill (LVF) with extensive flow-like features interpreted to be glacial in origin. We have modeled this deposit using glacial flow models linked to atmospheric general circulation models (GCM) for conditions consistent with the deposition of snow and ice in amounts sufficient to explain the interpreted glaciation. In the first glacial flow model simulation, sources were modeled in the alcoves only and were found to be consistent with the alpine valley glaciation interpretation for various environments of flow in the system. These results supported the interpretation of the observed LVF deposits as resulting from initial ice accumulation in the alcoves, accompanied by debris cover that led to advancing alpine glacial landsystems to the extent observed today, with preservation of their flow texture and the underlying ice during downwasting in the waning stages of glaciation. In the second glacial flow model simulation, the regional accumulation patterns predicted by a GCM linked to simulation of a glacial period were used. This glacial flow model simulation produced a much wider region of thick ice accumulation, and significant glaciation on the plateaus and in the regional plains surrounding the dichotomy boundary. Deglaciation produced decreasing ice thicknesses, with flow centered on the fretted valleys. As plateaus lost ice, scarps and cliffs of the valley and dichotomy boundary walls were exposed, providing considerable potential for the production of a rock debris cover that could preserve the underlying ice and the surface flow patterns seen today. In this model, the lineated valley fill and lobate debris aprons were the product of final retreat and downwasting of a much larger, regional glacial landsystem, rather than representing the maximum extent of an alpine valley glacial landsystem. These results favor the interpretation that periods of mid-latitude glaciation were characterized by extensive plateau and plains ice cover, rather than being restricted to alcoves and adjacent valleys, and that the observed lineated valley fill and lobate debris aprons represent debris-covered residual remnants of a once more extensive glaciation. 相似文献
16.
《Icarus》1986,68(2):187-216
Mars had outgassed at least 0.5 to 1 km of water, 10 to 20 bar of CO2, and 0.1 to 0.3 bar of N2. The volatiles that have been retained are mostly in the cratered uplands. Terrain softening, fretted channels, debris flows, and closed depressions indicate that at least the upper 2 km of the cratered uplands at high latitudes (>30°) contain ice in amounts that exceed the porosity, estimated to be 10–20%. Theoretical studies, and lack of these features in the cratered uplands at low latitudes, suggest that the upper 1 km of the uplands at low latitudes is ice poor. However, valley networks indicate that water was present near the surface early in the planet's history, although in amounts smaller than at high latitudes. The entire upper 1 km, planetwide is estimated to have contained 75–125 m of water at the end of heavy bombardment. The largest sink for water is the megaregolith below 1 km. Episodic eruption of water from the deep megaregolith cut many of the large outflow channels. From the volume of water needed to cut the circum-Chryse channels, and assuming uniform planetwide distribution of water, the deep megaregolith is estimated to have contained at least 350 m of water at the end of heavy bombardment, thereby giving a total minimum inventory of 424–475 m planetwide. Most of the water lost from the low-latitude uplands by diffusion and in cutting the valley networks is now believed to be in the polar layered terrains. Most of the water involved in cutting the outflow channels is in the low-lying northern plains where a variety of features that have been attributed to ground ice is present. A large fraction of the planet's surface has been overplated with water-poor volcanics, of which we have samples in the SNC meteorites. The younger volcanics have reacted extensively with the old volatile-rich basement. Some of the CO2 and N2 outgassed was lost during heavy bombardment by impact erosion of the atmosphere and other processes. The remaining was fixed carbonates and folded deep into the megaregolith during heavy bombardment. 相似文献
17.
We address key factors involved in determining water flow conditions in outflow channels on Mars, including the temperature of the sub-surface water being released and the environmental conditions of low temperature, low atmospheric pressure, and low acceleration due to gravity. We suggest how some of the assumptions made in previous work may be improved. Our model considers the thermodynamic effects of simultaneous evaporation and freezing of water, and fluid dynamical processes including changes in flow rheology caused by assimilation of cold rock and ice eroded at the channel bed, and ice crystal growth due to water freezing. We model how far initially turbulent water could flow in a channel before it erodes and entrains enough material to become laminar, and subsequently ceases to erode the bed. An ice raft will begin to form on the flood while transition occurs between turbulent and laminar flow. Estimates are given for water transit times, ～17–19 h, initial water depths, 50–62 m, and average flow speeds, 5–12 m s?1, in the Mangala and Athabasca Valles. We show that these two outflow channels, and by implication others like them, could plausibly have been formed in single water release events. Resulting mean erosion rates are approximately 0.7 mm s?1, a factor of three greater than previous estimates based on combinations of estimates of flood duration and required water volumes. This is explained by the consideration of the effects of eroded ice and the physics of thermal erosion in the present study. 相似文献
18.
Kent D. Trego 《Earth, Moon, and Planets》1985,33(1):103-104
If we assume that the Martian outflow channels are result of sporadic melting of ground ice, their planet-wide distribution could imply that a sheet of ice once covered Mars. This ice sheet could have acted, in a similar manner as Hoyle's oceanic meteoric dust suspension layer model as an initiator of a Martian ice age which would be responsible for the decline of valley network formation at the end of the heavy bombardment period. 相似文献
19.
Crofton B. Farmer 《Icarus》1976,28(2):279-289
The factors which affect fusion and evaporation of ice under a variety of Martian surface conditions are examined. It is found that a frost or ice deposit will pass through a transient liquid phase in temperate latitudes during summer, if the ice is partly or wholly dust covered. The barrier to free gaseous diffusion which the surface material presents is, under favorable (and definable) conditions, more than adequate to force the water to remain in the liquid state until its evaporation is complete. Furthermore, for a realistic range of regolith particle sizes and porosities, and depths of burial of the ice, the lifetime of the ice can be considerably longer than the duration of a single diurnal warming cycle. Current knowledge of the seosonal and diurnal behavior of the atmospheric vapor is summarized and discussed as it relates to the availability of surface ice at temperate latitudes. 相似文献
20.
L. Colangeli J.J. Lopez-Moreno P. Nørnberg V. Della Corte F. Esposito E. Mazzotta Epifani J. Merrison C. Molfese P. Palumbo J.F. Rodriguez-Gomez A. Rotundi G. Visconti J.C. Zarnecki 《Planetary and Space Science》2009,57(8-9):1043-1049
Dust and water vapour are fundamental components of the Martian atmosphere. In view of tracing the past environmental conditions on Mars, that possibly favoured the appearing of life forms, it is important to study the present climate and its evolution. Here dust and water vapour have (and have had) strong influence. Of major scientific interest is the quantity and physical, chemical and electrical properties of dust and the abundance of water vapour dispersed in the atmosphere and their exchange with the surface. Moreover, in view of the exploration of the planet with automated systems and in the future by manned missions, it is of primary importance to analyse the hazards linked to these environmental factors. The Martian Environmental Dust Systematic Analyser (MEDUSA) experiment, included in the scientific payload of the ESA ExoMars mission, accommodates a complement of sensors, based on optical detection and cumulative mass deposition, that aims to study dust and water vapour in the lower Martian atmosphere. The goals are to study, for the first time, in-situ and quantitatively, physical properties of the airborne dust, including the cumulative dust mass flux, the dust deposition rate, the physical and electrification properties, the size distribution of sampled particles and the atmospheric water vapour abundance versus time. 相似文献